Many regions of the world suffer from high concentrations of heavy metals, which pollute aquifers and water bodies. Historical and continued use of heavy metals in industrial processes continues to plague the world as an anthropogenic source of metal pollution. Anthropogenic activities such as mining, industry, and dredge material containment facilities can contribute to heavy metal pollution. Contaminated soils and waters, produced by either direct leakage or indirect leaching of wastes, will themselves leach wastes to surrounding areas. These pollutants, when ingested in a bioavailable form, are mutagenic/genotoxic. Heavy metals that accumulate in biological material poison metabolic pathways including glycolysis; oxidative phosphorylation; and protein, carbohydrate, and lipid metabolism. Active stewardship, critical for environmental health, directs the remediation of dangerously contaminated environments.
Present technologies for removal of these pollutants include both in-situ and ex-situ chemical treatment and physical sequestering. Ex-situ treatment of metal pollution involves pumping the contaminated water from storage to a treatment facility where methods such as ion exchange, chelation, and agglutination are applied. In-situ methods, such as Permeable Reactive Barriers, use materials such as zero-valent iron, phosphate and carbonate minerals, coal fly-ash, and blast furnace oxygen slag to remove pollutants. Although there typically is no need to forcefully move materials to be cleaned with in-situ methods, the contaminants must somehow contact the chemical or matrix responsible for their remediation.
The Department of Energy Hanford Site, Savannah River, and Iron Mountain, Calif. are just a few examples of contaminated sites in the United States. The leaching of high concentrations of heavy metals is a detriment to the environment and often contaminates drinking water sources. In many cases, soil and groundwater is frequently contaminated with a mix of cationic (i.e., Pb, Zn, Cu) and anionic (i.e., As, Cr) metals making remediation technically difficult.
The use of amendments, such as lime, sulfide salts, or ferric chloride injections, is a common remediation technique. Current amendments usually change contaminant speciation by altering site conditions. The changes in the environment can convert the site back to its pre-amendment state, re-releasing the contaminants. Amendments like ferric chloride injections, which work primarily by decreasing pH in an attempt to convert As (III) to As (V), can mobilize previously stable cationic metals.
In some instances, current technologies may present various challenges. For example, among other issues, the costs of treatment media may be high. Furthermore, after remediation the cleanup media themselves are hazardous wastes, and/or the sequestered material(s) may be released after time or an environmental change. Still further, treatments such as chemical oxidation may immobilize one hazardous material but release another.